Abstract 1015: mTOR inhibitor induces non-apoptotic, autophagy dependent cell death (ADCD) in TP53 mutant HNSCC

Abstract

Abstract TP53 is the most frequently mutated gene in Head and Neck Squamous Cell Carcinoma (HNSCC). Patients with HPV-negative TP53 mutant HNSCC have the worst prognosis. A phase II multi-institutional clinical trial, determined that, adjuvant therapy with an mTOR inhibitor significantly increased 2-year progression free survival (PFS) in patients with p53 mutant tumors. Previous studies have shown that mutant p53 causes sustained activation of PI3K/AKT/mTOR pathway in cancer. We investigated the mechanism by which the mTOR inhibitor CCI-779 inhibits cell growth of TP53 mutant HNSCC in vitro. HPV-negative HNSCC cell lines harboring TP53 mutations at different positions were treated with CCI-779.CCI-779 significantly reduced cell viability and colony forming ability of all the cell lines tested. CCI-779 did not induce apoptosis or cell cycle arrest in these cells. However, the reduction in cell viability was associated with increase in autophagy as demonstrated by increased levels of LC3-II. The autophagy inhibitors, 3-methyl adenine (3-MA) and ULK-101 restored the cell viability by inhibiting CCI-779 induced autophagy. Therefore, we postulate that CCI-779 induces nonapototic autophagy dependent cell death (ADCD) in HPV-negative TP53 mutant HNSCC cell lines. CCI-779 induced autophagy is accompanied by increased level of ULK1 and Beclin1 as well as increased phosphorylation of ULK1 at Serine 555. However, ULK101 (ULK1 inhibitor) and 3-MA (PI3K inhibitor) when combined with CCI-779 reduced the level of both total and phosphorylated levels of ULK1, which resulted in inhibition of autophagy and improved cell viability. However, combination of CCI-779 and bafilomycin A1 treatment showed increased level of LC3-II, ULK1 and phospho-ULK1 (S555) with concomitant decrease in cell viability. Our data suggests that rapalog induces ADCD by facilitating autophagy initiation through the stabilization and activation of ULK1. Moreover our data demonstrated that CCI-779 mediated activation of ULK1 is associated with down regulation of mTORC1 but not mTORC2 signaling. This is a novel mechanism of how rapalogues induce ADCD through the stabilization/activation of ULK1 in HPV-negative TP53 mutant HNSCC cells. Together these data highlight the importance of how mTORi can be used to accelerate tumor cell death in HPV-negative TP53 mutant HNSCC. Citation Format: Md Maksudul Alam, Janmaris Marin Fermin, Patrick T. Spiller, Channing Dorr, Xiaohua Rong, Tara-Moore Medlin, Alok R. Khandelwal, Cherie-Ann Nathan. mTOR inhibitor induces non-apoptotic, autophagy dependent cell death (ADCD) in TP53 mutant HNSCC [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1015.